Steam generator recirculating pump operation



p 17, 1968 D. PALCHIK 3,401,674

STEAM GENERATOR RECIRCULATING PUMP OPERATION Filed Sept. 20, 1966 3Sheets-Sheet l FIG-1 INVENTOR DAV/D PALCH/K AGENT United States Patent3,401,674 STEAM GENERATOR RECIRCULATING PUMP QPERATION David Palcliik,Bloomfield, Conn., assignor to Combustion Engineering, Inc., Windsor,Conn., a corporation of Delaware Filed Sept. 20, 1966, Ser. No. 580,765Claims. (Cl. 122-406) ABSTRACT OF THE DISCLOSURE A once-throughsupercritical pressure steam generator of the recirculating type,employing a centrifugal pump to effect recirculation. A measure of thedensity of the fluid passing through the pump is determined and the pumpspeed is regulated in response to this measurement. The measure ofdensity may be the density itself, temperature, power consumption of thepump drive motor, or any other reliable parameter under the particularoperating conditions.

This invention relates to recirculating type supercritical pressuresteam generators and in particular to a method and apparatus foroperating a recirculating pump so as to obtain the maximum recirculationconsistent with pump drive power consumption limitations.

US. Patent 3,038,453 to W. H. Armacost illustrates a once-through steamgenerator with a recirculation system superimposed on the through-flowcircuit. The recirculating pump is located in the recirculating line sothat it withdraws fluid from a location downstream of the heatingsurface and reintroduces it upstream of the heating surface. This pumpis controlled to obtain a constant pressure drop across the heatingsurface.

US. Patent 3,135,252 to W. W. Schroedter also describes a once-throughboiler with a recirculation system superimposed on the through-flowcircuit. The recirculating pump used to produce or induce recirculationis a free-floating centrifugal pump. The free-floating pump on thesystem produces a generally uniform velocity of the fluid entering thewaterwall system over the operating range of the pump. This patentillustrates the pump as located not only in the recirculating line butalternately in the through-flow line in such a location that flow isinduced from a location downstream of the heat absorbing location to alocation upstream of the recirculating pump. In such a location the hotrecirculated fluid is mixed with the incoming feedwater so that thetemperature at the recirculating pump is lower than the temperature ofthe fluid naturally being recirculated.

The temperature of the fluid entering the Waterwall circuits variessubstantially over the load range even though the velocity of the fluidis approximately constant. This difference in temperature results in aconsiderable change in density so that the mass flow of the waterentering the waterwall circuits is considerably reduced at low loadswhere high temperatures exist at the pump. Since the resistance of theinternal film inside the boiler tubes to transfer heat is a functionbasically of the mass flow rate rather than velocity, a relatively poorconductance results when high temperatures exist at the waterwall inlet.Even though relatively low average absorption rates exist at this time,local rates may be relatively high with resultant high metaltemperatures. It is desirable to increase the mass flow rate at thistime to increase the factor of safety in the design of the boilertubing.

It is an object of my invention to provide a method and apparatus foroperating a recirculating pump which is located intermediate the mixingvessel and the waterwall heating surface in such a manner as to obtainthe maxirnum recirculation consistent with the circulating pump drivepower consumption limitations.

Other objects and advantages of the invention will become apparent tothose skilled in the art as the description proceeds.

With the aforementioned objects in view, the invention comprises anarrangement, construction and combination of the elements of theinventive organization in such a manner as to attain the resultsdesired, as hereinafter more particularly set forth in the followingdetailed description of an illustrative embodiment, said embodimentbeing shown by the accompanying drawings wherein:

FIG. 1 illustrates a once-through recirculating type steam generatorhaving a mixed flow pump with the pump speed being controlled by use ofa motor generator set in a fluid coupling, and the control beingresponsive to the power consumption of the motor driving therecirculating p p;

FIG. 2 is a curve illustrating velocity conditions entering thewaterwall with constant pump speed;

FIG. 3 is a curve illustrating the mass flow rates at the waterwallinlet for different pump speeds;

FIG. 4 illustrates the kilowatt input to the motor for various pumpspeeds;

FIG. 5 illustrates the motor revolutions per minute for a constantkilowatt input; and

FIG. 6 illustrates a steam generator wherein the variable speed motordriving the circulating pump is controlled in response to thetemperature of the fluid passing through the circulating pump.

Water from the hot well 2 passes through feedwater line 4 which includesthe condensate pump and low pressure feedwater heaters (not shown).Feedwater pump 6 raises the pressure of the water to about 4000 p.s.i.with the water then flowing to the steam generator through feedwatervalve 8.

This high pressure water is passed through the economizer 10 to themixing vessel 12 and continues through the circulating pump 14 and thefurnace wall circuits 16. These furnace wall circuits are in the form ofa plurality of parallel tubes which line the walls of a radiant furnace.Fuel burner 18 fires coal or oil into the furnace for combustion withthe heat being radiantly transmitted to the furnace wall circuits 16.

The through-flow of water continues through the furnace wall outlet line20 and the superheater 22 to the main steam line 24. Combustion gasesformed by the combustion form burner 18 pass over the surface ofsuperheater 22 heating the steam passing through that surface as well asover the surface of economizer 10 thereby heating the water passingthrough that surface.

Turbine control valve 26 controls the steam flow through steam line 24to turbine 28. This turbine is directly connected to electric generator30 for the generation of power. Steam is exhausted from the turbine 28and condensed in the condenser 32 so that it returns to the hot well 2.The water is then again recycled as in the conventional power plantcycle.

Recirculating line 34 is connected to withdraw a portion of thethrough-flow from the furnace wall outlet line 20 and return thisportion to the mixing vessel 12 which is located upstream of the furnacewall circuit 16. The circulating pump 14 adds pressure energy to theWater passing therethrough, and in conjunction with the recirculatingline 34 operates to maintain a flow in the furnace wall circuits higherthan the normal throughflovv as explained in US. Patent 3,135,252 to W.W. Schroedter. This recirculating line includes the stop check valve 36which operates to prevent reverse flow in the recirculating line.

The circulating pump 14 is of the conventional centrifugal type havingan impeller which is rotated at 1800 r.p.m. Since the pressure of thewater at this'location is about 3900 p.s.i., it is difficult to properlyseal a shaft passing to the pump from an external motor. Accordingly, amotor of the canned type is used to drive this pump with the rotor ofthe motor being contained within a thin metallic structure. This rotoris of the squirrel cage design With no electrical connections to therotor.

With this motor driving the pump at constant speed, the velocitycharacteristic is as indicated by curve 38 in FIG. 2 and more fullydescribed in US. Patent 3,135,252 to W. W. Schroedter. In this curve theordinate indicates velocity in feet per second of the fluid entering thetubes of the furnace wall section 16. The abscissa generally indicatesthe quantity of through-flow through the steam generator as a percentageof design full load flow. Since these units are generally started up byinitiating flow at percent While the steam generator is cold and thenfiring to warm up the unit before increasing flow, the 10 percentcondition is shown at two locations along the abscissa. The firstcondition indicates 10 percent flow with cold water in the steamgenerator while the second 10 percent point indicates the conditionswith the furnace wall circuit up to full temperature (in the order of800 F. at the outlet). It can be seen that the velocity entering thewaterwalls is essentially constant throughout the range in which thepump operates. Curve 40 indicates the velocity without recirculation. At78 percent load the output of the pump is insufficient to inducerecirculation and from this point on the through-flow alone causes the'velocity to increase.

The solid line 44 in FIG. 3 indicates the mass flow characteristics atconstant pump speed of 1800 r.p.m. throughout the same operating range.The ordinate in this case is the mass flow of the fluid entering thefurnace wall tubes expressed in pounds per hour per square feet. It canbe seen that at the 10 percent through-flow condition as the Water isheated up, while the velocity increases slightly, the mass flow ratedecreases substantially. At increasing through-flows a mass flow rategenerally increases again.

FIG. 4 using the same abscissa indicates the kilowatt input to thecanned motor in curve 46 for the same 1800 r.p.m. impeller speed. Due tothe natural characteristic of centrifugal pumps, the kilowatt inputdecreases as the water density decreases and subsequently increases in amanner similar to the mass flow.

Curves 48 and 50 in FIG. 3 show the mass flow entering the furnace wallcircuits for impeller speeds of 2250 and 2700 r.p.m. respectively.Similar curves are illustrated in FIG. 4 where curve 52 shows thekilowatt input for 2250 r.p.m. and curve 54 the kilowatt input at 2700r.p.m. While the curves are shown for only a few of the impeller speeds,obviously there are an infinite number of speeds which could be used anda corresponding infinite number of curves. An inspection of FIGS. 3 and4 will show that when the mass flow is low, the kilowatt input for agiven impeller speed is also low. Therefore in order to improve the massflow conditions, the impeller speed is increased where the mass flowincreases. It, however, is desirable to limit the kilowatt input to themotor. This kilowatt input affects not only the cooling requirements ofthe motor but also the power plant switch gear and transformer capacitywhich must be installed to supply the energy requirements for the pump.In FIG. 4 line 56 represents a constant kilowatt input to the motordriving the circulating pump. By interpolating between curves 44, 48 and50 the appropriate r.p.m. can be obtained to maintain this constantkilowatt input. This motor speed is shown by curve 57 in FIG. 5. Itseffect on the mass flow is illustrated by line 58 shown on FIG. 3. Itcan be seen that by operating the circulating pump in such a manner theparticularly low mass flow operating condition is avoided with noincrease in the power requirements of the pump. At all Conditions themaximum mass flow 4. is obtained compatible withpump motor powerlimitations.

In order to obtain this characteristiathe pump speed must be changed.Throttling of the flow through the pump or throttling of therecirculating flow will'not suflice, since this will not effectivelychange the power consumption of the motor driving the pump.

While the pump speed may be controlled manually,

FIG. 1 illustrates an .automatic control for maintaining the maximummass flow consistent with power consump-' tion requirements. Thecirculating pump 14 is driven by directly connected canned motor 60. Theconstant speed alternating current motor 62 is connected to the line andoperates to rotate alternating current generator 64 which is connectedthrough the fluid coupling 66. The output from the generator 64 passesto the motor 60 through Watt meter 68. By changing the frequencyofthecurrent' generated by generator 64, the speed of rotation of "thepump 14 is changed. Watt meter 68 senses the kilowatt consumption of themotor 60 and passes acontrol signal representing this power consumptionthrough control line 70. At set point 72 this signal is compared to asignal representing the desired kilowatt consumption with the errorsignal passing through control line 74 to controller 76. This controllerthen operates on the fluid coupling 66 changing the speed of thegenerator 64 and therefore the speed of motor 60. This is automaticallyreadjusted until pump 14 is operating at such a speed that the kilowattconsumption remains at the desired value. This control circuit could bearranged in such a manner that the kilowatt speed of the motor ischanged in steps, allowing some deviation from the desired kilowattconsumption rather than using the exact approach illustrated here.

FIG. 6 illustrates a power plant which is identical with thatillustrated in FIG. 1 except for the method of controlling the speed ofthe circulating pump 14. The density of the fluid flowing through thepump is affected by temperature at a constant pressure. In mostcircumstances nominal variations of pressure do not significantly affectthe density. Therefore when operating in the usual pressure range,temperature alone may be used as an indication of the density. Thekilowatt consumption of the motor driving a circulating pump is directlyproportional to the density of the fluid being pumped. Therefore, thistemperature may be used as an indication of the kilowatt consumption ofthe motor. A variable speed motor which drives the circulating pump 14is of the two-speed two-winding type. This motor will operate at either1800 r.p.m. or 2700 r.p.m. This motor is connected through controller 82to the line power supply. This controller is operative to activate themotor 80 selectively on the 1800 or 2700 r.p.m. windings.

Temperature transmitter 84 senses the temperature of the fluid passingthrough the conduit 13 and the circulating pump 14 and sends a signalindicative of this temperature through control line 86 to the controller82. As indicated before, this temperature signal is a function of thedensity of the fluid being pumped. When the temperature of this fluiddrops below 540 F., the controller 82 operates to activate the 1800r.p.m. windings of pump 80. When the temperature rises to about 550 F.,the controller operates to increase the speed of the motor 80 to 2700r.p.m. With various multiple winding motors, this operation may be usedto select any desired steps in motor speed. Where pressure varies tosuch. an extent and in such a range that it significantly affects thedensity of the fluid being pumped, the pressure transmitter must also beincluded so that the pressure and temperature signals are both used todetermine a signal representative of the density of the fluidpassingthrough the motor.

While I have illustrated and described a preferred ;embodiment of myinvention is Y to be understood that such is merely illustrative and notrestrictiVeJand that variations and modifications may be made thereinwithout departing from the spirit and scope of the invention.

I therefore do not wish to be limited to the precise details set forthbut desire to avail myself of such changes as fall within the purview ofmy invention.

What I claim is:

1. A supercritical pressure steam generator comprising: a heat exchangesection; means for establishing a throughflow of water through said heatexchange section; a mixing zone upstream of said heat exchange sectionand conduit means for conveying fluid from said mixing zone to said heatexchange section; means for withdrawing a portion of the throughflowfrom a location downstream of said heat exchange section andreintroducing said portion in said mixing zone; a centrifugal pumphaving an impeller located in said conduit means; driving means forrotating the impeller of said centrifugal pump; sensing means forsensing a measure of the density of the fluid as it passes through saidconduit means and centrifugal pump; means for varying the speed ofrotation of said impeller in response to said sensing means.

2. An apparatus as in claim 1 wherein said sensing means comprises meansfor sensing the power input to: said driving means.

3. An apparatus as in claim 1 having a furnace, and means for burningfuel within said furnace; said heat exchange section comprising tubeslining the Walls of said furnace.

4. An apparatus as in claim 3 wherein said driving means comprises acanned squirrel cage motor; the means for varying the speed of rotationof said pump comprising a constant speed electric motor, an alternatingcurrent electric generator, a fluid coupling connecting said alternatingcurrent motor and said alternating current generator, and electricalconnections for conveying the output of said alternating currentgenerator to said canned motor.

5. An apparatus as in claim 1 wherein said sensing means comprises meansfor sensing the temperature of the fluid passing through said conduitmeans.

6. A method of operating a supercritical pressure recirculating typesteam generator comprising: establishing a through-flow of Water;heating said through-flow to a temperature level by passing the water inheat exchange relationship with a heat source; forming a mixed flowportion of water flow by returning a portion of the through-flow at saidtemperature level to a location in said through-flow path upstream ofsaid heat exchange relation, and mixing the returned portion with thethrough-flow; increasing the pressure of said mixed flow portion bymechanically adding pressure energy to the mixed flow portion of waterflow; determining a measure of the density of said mixed flow portion ofthe water flow before heating the water flow; and regulating theincrease in pressure in response to said measure of density.

7. A method as in claim 6 wherein the measure of density of the mixedflow portion is determined by deter mining the energy required tomechanically add pressure energy to the mixed flow portion of Waterflow.

8. A method as in claim 7 wherein the increase in pressure is regulatedin response to the energy required, in such a manner as to maintain at aconstant value of energy required to mechanically add pressure energy.

9. An apparatus as in claim 6 wherein the measure of density of themixed flow portion is determined by measuring the temperature of themixed flow portion.

10. A supercritical pressure steam generator comprising: a heat exchangesection; means for establishing a through-flow of Water through saidheat exchange section; a mixing zone upstream of said heat exchangesection and conduit means for conveying fluid from said mixing zone tosaid heat exchange section; recirculating pipe means for withdrawing aportion of the through-flow from a location downstream of said heatexchange section and reintroducing said portion in said mixing zone;said recirculating pipe means and said conduit means comprising a firstportion of a recirculating loop; a centrifugal pump having an impellerlocated in said first portion; driving means for rotating the impellerof said centrifugal pump; sensing means for sensing a measure of thedensity of the fluid in the condition in which it passes through saidcentrifugal pump; means for varying the speed of rotation of saidimpeller in response to said sensing means.

References Cited UNITED STATES PATENTS 2,255,612 9/1941 Dickey 122451 XR2,324,513 7/1953 Junkins 12l2-451 XR 3,135,252 6/1964 Schroedter 1224063,194,219 7/1965 Henzalek 122--406 KENNETH W. SPRAGUE, Primary Examiner.

